Segregation of genetic material during mitosis is mediated by the microtubules of the mitotic spindle (see, e.g., McIntosh, in Microtubules, pp. 413-434 (Hyams & Lloyd, eds., 1994). During mitosis, chromosomes are dynamically attached to spindle microtubules via the kinetochore, which is a structure located at the centromere of the chromosome. Kinetochores are involved in coordinating chromosome movement via microtubule assembly and disassembly. The kinetochore and its component proteins thus play an important role in the dynamics of mitosis.
Spindle microtubules have a defined polarity, with their slow-growing, “minus” ends anchored at or near the spindle pole, and their dynamic, fast-growing “plus” ends interacting with chromosomes (McIntosh, et al., J. Cell Biol. 98:525-533 (1984)). During prometaphase, chromosomes establish interactions with the fast-growing plus ends of microtubules via the kinetochore. Chromosomes then undergo a series of microtubule-dependent movements, culminating in alignment at the metaphase plate, equidistant from the two spindle poles. This process is called “congression.” However, the molecular mechanisms underlying chromosome congression are poorly understood (see, e.g., Rieder, et al., J. Cell Biol. 124:223-33 (1994)). A major question has been whether any kinetochore-associated microtubule motors play an important role in congression.
The two predominant and opposing forces are currently thought to be responsible for chromosome movement during congression: (1) an anti-poleward polar force associated with regions of high microtubule density near the spindle poles, and (2) a poleward force generated at the kinetochore (Khodjakov, et al., J. Cell Biol. 135:315-327 (1996); Waters, et al., J. Cell Sci. 109:2823-2831 (1996); reviewed in Rieder, et al., Int. Rev. Cytol. 79:1-57 (1982); Mitchison, et al., Annu. Rev. Cell Biol. 4:527-49 (1988); Rieder, et al., J. Cell Biol. 124:223-33 (1994)).
Studies in vitro have demonstrated the presence of both plus and minus end-directed microtubule motor activities on kinetochores that may be responsible for these chromosome movements (Mitchison, et al., J. Cell Biol. 101:766-77 (1985); Hyman, et al., Nature 351:206-211 (1991)). The outstanding issue, however, has been the identity of the molecules at the kinetochore which act as motors and generate the force for chromosome movement.
In general, both genetic and biochemical approaches have demonstrated crucial roles for microtubule motors in spindle assembly, spindle pole separation, and regulation of spindle microtubule dynamics. These motors include Eg5, CHO1/MKlp1, ncd, cut7, bimC, CIN8, KIP1, KAR3, Xklp2, XKCM1, and XCTK2 (Sawin, et al., Nature 359:540-543 (1992); Blangy, et al., Cell 83:1159-1169 (1995); Sawin, et al., J. Cell Biol. 112:925-940 (1991); Nislow, et al., J. Cell Biol. 111:511-522 (1990); Endow, et al., J. Cell Sci. 107:859-867 (1994); Hagan, et al., Nature 347:563-566 (1990); Hagan, et al., Nature 356:74-76 (1992); Enos, et al., Cell 60:1019-1027 (1990); Hoyt, et al., J. Cell Biol. 118:109-120; Roof, et al., J. Cell Biol. 118:95-108 (1992); Saunders, et al., Cell 70:451-458 (1992), Boleti, et al., J. Cell. Biol. 125:1303-1312; Walczak, et al., Cell 84:37-47 (1996); Walczak, et al., J. Cell Biol. 136:859-70 (1997)). Two kinesin superfamily members, Xenopus Xklp1 and Drosophila nod localize to chromosome arms. With the exception of these two chromatin-associated motors, which are thought to mediate polar ejection forces, none of these other proteins have been implicated directly in congression or in chromosome movement during other phases of mitosis (Theurkauf, et al., J. Cell Biol. 116:1167-1180 (1992); Afshar, et al., Cell 81:129, Cell 81:128-138 (1995); Vernos, et al., Trends in Cell Biol. 5:297-301 (1995)).
A candidate for powering chromosome movement in mitosis is centromere-associated protein-E (CENP-E), a member of the kinesin superfamily of microtubule motor proteins. Human CENP-E has been cloned and is an integral component of the kinetochore (Yen, et al., Nature 359:536-539 (1992); Yao, et al., The microtubule motor CENP-E is an integral component of kinetochore corona fibers that link centromeres to spindle microtubules (manuscript)). CENP-E localizes to kinetochores throughout all phases of mitotic chromosome movement (early prometaphase through anaphase A) (Yen, et al., Nature 359:536-539 (1992); Brown, et al., J. Cell. Biol. 125:1303-1312 (1994); Lombillo, et al., J. Cell Biol. 128:107-115 (1995)).
Previous efforts have suggested a role for CENP-E in mitosis. Microinjection of a monoclonal antibody directed against CENP-E into cultured human cells delays anaphase onset (Yen, et al., EMBO J. 10:1245-1254 (1991)). Anti-CENP-E antibody injection into maturing mouse oocytes induces arrest at the first reductional division of meiosis (Duesbery, et al., Proc. Natl. Acad. Sci. USA (in press, 1997)). Antibodies against CENP-E block microtubule depolymerization-dependent minus end-directed movement of purified chromosomes in vitro (Lombillo, et al., J. Cell Biol. 128:107-115 (1995)).
However, these experiments have not demonstrated the precise role of CENP-E in mitosis, nor have they shown the activity of CENP-E, in particular any motor activity. Recently, CENP-E was reported to be associated with minus end-directed microtubule motor activity, raising the possibility that CENP-E might be responsible for poleward kinetochore movements (Thrower, et al., EMBO J. 14:918-926 (1995)). However, biologically active CENP-E has never been isolated, neither from naturally occurring nor recombinant sources.